1) Field of the Invention
The present invention relates to friction stir welding and, more particularly, to positioning a structural assembly for friction stir welding, and friction stir welding the structural assembly to form a unitized friction stir welded structure.
2) Description of Related Art
Friction stir welding is a process in which a rotating tool, such as a pin or probe, is urged into and/or through a workpiece, e.g., to join multiple members of the workpiece in a solid state or to repair cracks in a workpiece. Typically, the pin extends from a shoulder, which can be flat, concave, or otherwise contoured, and the shoulder is urged against the workpiece so that the pin is urged therein. The pin is then urged through the workpiece to form a continuous weld joint. For example, during one conventional friction stir welding process, the probe of the rotating tool is plunged into a workpiece or between two workpieces by a friction stir welding machine to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. The tool can be tilted approximately 3° relative to the workpiece such that the trailing edge of the shoulder is thrust into and consolidates the plasticized material. Upon solidification of the plasticized material, the members of the workpiece are joined along the weld joint. The magnitude of force exerted by the friction stir welding tool must be maintained above a prescribed minimum in order to generate the required frictional heating. Friction stir welding is further described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference.
Friction stir welding has been demonstrated to be a successful joining method for forming certain types of joints, such as the butt joint where the probe is inserted within abutting edges of two structural members, or a lap joint in which the probe is inserted in a direction substantially perpendicular to the interface between overlapping structural members. In addition, other types of friction welding have also been shown to be useful for forming certain joints.
Common aircraft wing structure includes upper and lower wing skins with hundreds or thousands of moldline fasteners joining the two. Reduction or elimination of these fasteners is desirable for cost reduction (fewer fasteners equates to less cost to drill holes and install fasteners), surface finish (a smooth surface provides fewer reflecting surfaces), and durability (fewer fasteners reduces crack initiation sites and fuel leak paths). One way to reduce the number of fasteners in structures with metallic skins and substructure is to friction stir weld the skin to the spars and ribs from the outer moldline. A lap joint is typically formed between the skin and spars and/or ribs, which produces several drawbacks. For example, lap joints contain high natural stress concentrations, and may trap moisture, which may lead to crevice corrosion problems. Because of their location and geometry, lap joints typically see high tension and peel stresses. Forming lap joints also requires that the substructure resist the forces applied during friction stir welding, which can require the substructure to be stiffer than actually needed for service loads, leading to excess weight. Finally, lap joints can leave a rough surface behind that is undesirable from a surface finish standpoint.
It would therefore be advantageous to provide an apparatus and system for friction stir welding a structural assembly that is relatively inexpensive and effective. It would also be advantageous to provide a structural assembly with a reduced number of fasteners and a desirable surface finish. It would further be advantageous to provide an assembly that promotes efficient friction stir welding of the structural assembly.